LED electrostatic breakdown principle
LEDs based on PN junction structure are inductively charged during the manufacturing, screening, testing, packaging, storage and transportation, and installation and use. If not released in time, the higher voltage developed on the two electrodes of the LED will be directly applied to the ends of the PN junction of the LED chip .
When the voltage exceeds the maximum withstand value of the LED, the electrostatic charge will discharge between the two electrodes of the LED chip in a very short instant (nanosecond level). The heat of the power joule will cause the conductive layer inside the LED chip and PN to emit light. Part of the layer forms a high temperature, which will melt the layers into small holes, causing leakage and short circuit.
LED antistatic features
The PN junction is the basic structure of the LED, but its antistatic ability is different due to the different materials.
The red LED has strong antistatic ability. The materials used for red, orange and yellow LEDs are mainly GaP and GaAs and their mixed crystal GaAsP. These compound semiconductors have a band gap of 1.8-2.2 eV. The PN junction is easily doped with a low-resistance material, and its conductivity is good. When it encounters an electrostatic charge, it can be easily released, so the antistatic ability is better.
Green and blue LEDs have weak antistatic ability
The material near the PN junction of the green and blue LEDs is InGaN or AlGaN, GaN, and the forbidden band width is 3.3 eV, which is about 50% larger than that of the general red, orange, and yellow LED materials, and the resistivity is relatively high. In addition, the substrate of this type of LED is made of high-resistance sapphire (Al2O3) or silicon carbide (SiC), which has poor conductivity and thermal conductivity. For example, the PN junction electrode of the blue LED of the Al2O3 substrate is a V-type electrode (commonly known as a two-electrode type), and the distance between the electrodes is <300 μm. Once the induced electrostatic charge is accumulated, it is easy to generate a self-excited discharge there. Also, since the light-emitting starting layer of AlGaNlGaN is thin, the layer is more likely to be broken down in electrostatic discharge.
InGaN material with SiC as the substrate is basically an L-type electrode vertical structure (single-electrode type, such as CREE ) LED, its anti-static voltage mechanical mode reaches 600V or more, and human body mode is up to 5000V (although CREE officially nominal 1000V) ). The LED device generally based on Al2O3 is usually a V-type electrode, and its antistatic voltage mechanical mode is only about 400V or lower.
Lights are more susceptible to electrostatic damage than chips
All processes of LED chip production from the subsequent packaging of LED lamps and the production of LED applications have been threatened by static electricity, but the packaged LED lamps are much less likely to be damaged by static electricity than the chips.
The specific reason is: the size of the chip is very small, such as 12 mil chip, its size is about 304m Χ 304μmm, and the distance between the electrodes is smaller (generally less than 100μm), if such a small LED chip is in the electrostatic field The potential difference of such a small electric field spacing is close to zero, and a high electrostatic voltage is not formed, and generally no electrostatic damage occurs unless the antistatic ability of the chip is extremely poor; in addition, the tiny area of ​​the LED chip electrode is more Limits the possibility of LED chip electrodes contacting electrostatic discharge.
The packaged LED lamp generally has a spacing of about 2 mm, which is 20 times longer than the chip electrode. It is more likely to generate a higher voltage in the electrostatic field than in the electrostatic field.
Therefore, the probability of the chip being damaged by static electricity is much smaller than that of the LED lamp. The electrostatic protection measures for the LED lamp should be paid more attention. The electrostatic evaluation of the anti-static LED chip in the form of a packaged lamp is more realistic. .
Stronger anti-static ability than forward
When static electricity discharges the LED in reverse, the current is more concentrated than the forward discharge, and the power density is larger. Therefore, the ESD failure threshold of the LED reverse discharge is much lower than the forward direction, that is, the voltage of the LED reversely receiving static electricity is much lower. Therefore, it is most reasonable to evaluate the antistatic performance of LEDs against their antistatic properties.
Common phenomenon after LED electrostatic breakdown
Because the mechanism of electrostatic breakdown of LED is that the Joule heat generated by the instantaneous high voltage is applied to the PN junction of the chip at a very high power density, thereby melting the chip, generally can burn a 'pit' inside the chip, so The LED appears after being completely and electrically broken:
The most obvious is that the reverse leakage current of the LED increases, and even the two ends of the LED are close to a short circuit (about 10-30 ohms);
Other phenomena are - the brightness of the LED is greatly reduced, or even not at all;
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